1. Field of the Invention
The present invention relates to an optical communication module. More particularly, the present invention relates to an optical demultiplexer having at least one Bragg diffraction grating for minimizing crosstalk on the sides of transmitting and receiving optical signals, as well as an optical communication module using the optical demultiplexer.
2. Description of the Related Art
Recently, there has been increased demand for a bi-directional optical communication module, in which two or more optical signals of different wavelengths are allowed to travel in either direction through a single optical fiber, so that they can be transmitted or received.
FIG. 1 illustrates a principle of wavelength multiplexing during bi-directional optical communication using light of different wavelengths.
Referring to FIG. 1, one base station 1 is connected with one subscriber unit 2 through one optical fiber 3. So as to not obscure the drawing with needless detail, only one subscriber unit 2 although typically there would be many such units. Thus, in an actual bi-directional optical communication unit there are many branching points, from which each optical fiber extends to reach each subscriber unit. The base station 1 drives a light emitting element, i.e., a laser diode LD1 (4) by converting information, such as a telephone signal, a television signal, etc. into a digital signal. This digital signal, identified as having a wavelength of λ1, is transmitted through a demultiplexer 5 of the base station to the optical fiber 3, and then is received through a demultiplexer 6 of the subscriber unit into a light receiving element, i.e., a photo diode PD2 (7). Subsequently, the received optical signal is converted into an electrical signal again, and then is regenerated into an Internet text or a moving picture. In this manner, a direction traveling from the base station 1 toward the subscriber unit 2 is called a “downstream” direction.
Meanwhile, on the side of the subscriber unit 2, a data information signal is converted into an optical signal having a wavelength identified as λ2 by a laser diode 8. Light of λ2 is inputted into a photo diode PD1 (9) through the demultiplexer 6, the optical fiber 3 and the demultiplexer 5 of the base station in that order. The base station 1 appropriately processes an electrical signal, which is converted by the photo diode PD1. In this manner, a direction from the subscriber unit 2 toward the base station 1 is called an “upstream” direction.
Moreover, in order to transmit optical signals having two or more wavelengths through a single optical fiber, both the base station and the subscriber unit require a function for recognizing the wavelengths and separating light paths. It is the demultiplexer that carries out this function.
A method of manufacturing a demultiplexer is exemplified by one using an optical fiber and one using an optical thin film waveguide. With regard to the use of an optical thin film waveguide, recent trends are that the waveguide is sought to have a compact size, be low in price and have a high degree of integration. An example of a demultiplexer using an optical thin film waveguide is exemplified by a wavelength division multiplexing (WDM) filter, a multi-mode interferometer (MMI), or a directional coupler.
FIG. 2 shows a structure of an optical communication module according to the prior art, in which a demultiplexer using a wavelength division multiplexing filter is applied to the optical communication module. The demultiplexer is designed so that a multi-layer thin film 21 is inserted into a substrate 22, so that light entering from an optical fiber 23 and light exiting from a light source 24 are separated from each other according to a wavelength difference between them.
In FIG. 2, an optical signal having a particular wavelength of λ1 enters through the optical fiber 23, and then is propagated through a first waveguide to reach the multi-layer thin film 21 on the side away from the optical fiber. Since the multi-layer thin film 21 functions to reflect a particular wavelength of λ2, when light incidented from the optical fiber 23 has a transmission wavelength of λ1, not a reflection wavelength of λ2, the incident light transmits the multi-layer thin film 21 to reach an optical detector 25, for example, a photo diode that detects the incidented light. On the other hand, when light incidented from the optical fiber 23 has the reflection wavelength of λ2, for example, emitted from a laser diode, the incident light is then incidented into and propagated to a second waveguide, and then reflected by the multi-layer thin film 21 to exit through the optical fiber 23.
However, the prior art entails practical difficulties in actual application, because a process for inserting a multi-layer thin film into a substrate is very complicated, and such construction requires a highly sophisticated manufacturing method.
FIGS. 3 and 4 show constructions of a demultiplexer using an optical thin film waveguide in accordance with the prior art. Specifically, FIG. 3 shows a construction of a demultiplexer using a multi-mode interferometer (MMI). FIGS. 4 and 5 show constructions of a demultiplexer using a directional coupler (DC). More particularly, FIG. 4 shows a construction of a biplexer using two different wavelengths, and FIG. 5 shows a construction of a triplexer using three different wavelengths.
However, in the case of the demultiplexer using the conventional MMI shown in FIG. 3, crosstalk and insertion loss are changed depending on both a width W and a length L in a waveguide, as shown in FIGS. 6 and 7. FIGS. 6a and 6b show a change of the insertion loss O and the crosstalk P at wavelengths of 1550 nm and 1310 nm, respectively, according to a width W of a waveguide. FIGS. 7a and 7b show a change of the insertion loss O and the crosstalk P at wavelengths of 1550 nm and 1310 nm, respectively, according to a length L of a waveguide. In FIGS. 6 and 7, Q represents a design reference value. Here, the “crosstalk” refers to a ratio between a maximum output of a signal measured at an intended port and a maximum output of a signal measured at another certain port. Increase in the crosstalk means that distortion of signals become strong.
in FIGS. 4 and 5 show structures of directional couplers 401 and 501, respectively, wherein the crosstalk and insertion loss are also changed depending on a distance D between waveguides and a length L of a waveguide. FIGS. 8 to 10 graphically represent the changes in crosstalk and insertion loss.
FIGS. 8a and 8b show a change of the insertion loss O and the crosstalk P at wavelengths of 1550 nm and 1310 nm, respectively, according to a distance D between waveguides in the optical communication module employing the demultiplexer of FIG. 4. FIGS. 9a and 9b show a change of the insertion loss O and the crosstalk P at wavelengths of 1550 nm and 1310 nm, respectively, according to a length L of a waveguide. FIGS. 10a, 10b and 10c show a change of the insertion loss O and the crosstalks P1, P2 and P3 at wavelengths of 1550 nm, 1490 nm and 1310 nm, respectively, according to a distance D between waveguides in the optical communication module employing the demultiplexer of FIG. 5. In FIGS. 8a to 10c, Q represents a design reference value.
Thus, in the case of prior art demultiplxers, the satisfaction of a crosstalk standard requires a strict compliance with design optimal values in a width and a length of a waveguide, a distance between waveguides, and so on. A waveguide must be manufactured on the basis of this degree of compliance. However, according to a present process levels, it is difficult to avoid generating a tolerance of about ±0.2 μm. As a result, there is a problem in that a production yield of the demultiplexer is significantly reduced, as many of the manufactured demultiplexers are outside of the required tolerances.